1. Field of the Invention
The present invention relates to a power supply control circuit including a MOS-FET connected between a DC power supply and a load circuit and connected so that a body diode formed between the drain and the source is in the forward direction, and a holding circuit for maintaining the ON state of the MOS-FET for a predetermined time when the power supply from the DC power supply is stopped; and an electronic control device, a power supplying circuit, and a power supply control integrated circuit equipped with the power supply control circuit.
2. Description of the Related Art
When the polarity of the DC power supply such as battery is reverse connected by mistake in supplying power to electronic equipment such as an electronic control device mounted on the vehicle, the elements arranged in the electronic equipment may be damaged and the electronic equipment may breakdown. A protective circuit has been conventionally arranged to prevent such breakdown.
As shown in FIG. 1A, for example, a diode D100 is connected in series between a DC power supply 10 and a load circuit 31, which is the internal circuit of the electronic control device 30, so as to be forward biased, whereby the breakdown of the element of the load circuit 31 is avoided by shielding the reverse flow of the current by the diode D100 that becomes reverse biased when the polarity of the DC power supply 10 is reverse connected by mistake.
Furthermore, as shown in FIG. 1B, a current fuse 11 is connected in series between the DC power supply 10 and the load circuit 31, which is the internal circuit of the electronic control device 30, and a diode D200 is connected between the current fuse 11 and the load circuit 31 so as to be reverse biased, that is, so that the anode of the diode D200 is grounded, whereby the breakdown of the element of the load circuit 31 is avoided with a configuration in which the current fuse 11 is fused by the over current flowing in through the diode D200 that becomes forward biased when the polarity of the DC power supply 10 is reverse connected by mistake.
However, if the configuration shown in FIG. 1A is applied to the vehicle, the operating voltage to be supplied to the electronic control device 30 slightly lowers due to the voltage drop at the diode connected in series, and thus the start-up performance of the vehicle degrades such as when the capacity of the battery mounted on the vehicle lowers, and furthermore, the electronic control device 30 over heats by the heat generation of the diode when the diode is incorporated in the electronic control device 30 or when mounted close to the electronic control device 30.
In the configuration shown in FIG. 1B, the electronic control device 30 will not be supplied with power until replaced with a new current fuse 11 even if the connection to the DC power supply 10 is returned to the normal polarity after the current fuse 11 is fused by over current, and thus the function of the electronic control device 30 remains lost, but a usual driver cannot easily perform the task of replacing the current fuse 11. Furthermore, an absolute guarantee that the electronic control device 30 will not breakdown with respect to the current value and the time until the current fuse 11 is fused is difficult to make, and the electronic control device 30 may breakdown before the current fuse 11 is fused depending on the state of the power supply voltage or the impedance of the electronic control device 30.
In order to resolve such problems, a configuration is adopted in which a p-channel enhancement type MOS-FET 61 is connected as a switching element between the DC power supply 10 and the load circuit 31, which is the internal circuit of the electronic control device 30 so that the body diode D300 is in the forward direction, a Zener diode ZD1 is connected between the gate and the source of the MOS-FET 61 so that the anode is positioned at the gate, and the gate of the MOS-FET 61 is grounded by way of a resistor R1, as shown in FIG. 1C.
In this case, the MOS-FET 61 is turned ON and the load circuit 31 is supplied with power if the polarity of the DC power supply 10 is properly connected, but the MOS-FET 61 is turned OFF and the reverse flow of the current is shielded if the polarity of the DC power supply 10 is reverse connected, thereby avoiding the breakdown of the element of the load circuit 31.
However, even with the configuration shown in FIG. 1C, if a negative surge voltage that attempts to flow large current to the power supply line is generated when the MOS-FET 61 is in the OFF state, the possibility of the MOS-FET 61 being damaged is high since the withstanding voltage of the reverse voltage of the body diode D300 of the MOS-FET 61 is low compared to a commonly used stand-alone diode.
Thus, a power supply device arranged with a switching element that protects the internal circuit when the polarity of the DC power supply is reverse connected by mistake, and capable of handling the breakage of the MOS-FET 61 when the negative surge voltage is generated is proposed in Japanese Laid-Open Patent Publication No. 8-223935.
Such power supply device is configured with a switching element including an enhancement type MOS-FET connected between the DC power supply and the main circuit so that the body diode between the drain and the source is in the forward direction, and an active clamp circuit serving as a switching element protecting circuit configured by a Zener diode and a diode connected between the gate and the drain of the switching element, where the switching element is turned OFF to protect the main circuit when the polarity of the DC power supply is reverse connected by mistake, and the switching element is forcibly turned ON by the active clamp circuit when the negative surge voltage is applied.
However, the following problems arise even if the active clamp circuit described above is used.
The active clamp circuit is a circuit that allows the load energy, which is generated when the negative surge voltage is applied, to escape with the heat loss of the MOS-FET by using the MOS-FET in an unsaturated region, and intentionally creates a potential difference between the drain and the source of the MOS-FET to allow the load energy to escape with the loss energy caused by the potential difference.
The potential difference is determined by the reverse voltage of the Zener diode configuring the active clamp circuit, but the reverse bias voltage is usually set large since the load energy can be escaped at an earlier stage with larger potential difference.
Therefore, if the negative surge voltage is applied when using the power supply device, a large potential difference determined by the reverse bias voltage of the Zener diode configuring the active clamp circuit is created between the drain and the source of the MOS-FET, and thus the power consumption of the MOS-FET becomes large, that is, the heat generation becomes large due to the potential difference. Heat generation becomes larger since the MOS-FET is in the unsaturated state and is not completely in the ON state.
One factor that further adds to the problem of heat generation is field decay surge. The field decay surge is a negative surge energy that is generated according to the inductive load when the current is shielded from the state in which large current is flowing to the inductive load, and is known to be a very large voltage among the surge voltages generated in the vehicle. Since a great number of inductive loads of motor, solenoid coil, and the like that drives each part of the vehicle such as electrically operated power steering and wipers are arranged in the vehicle, the generation of the field decay surge is inevitable when the switch with respect to the inductive load that is being supplied with power from the power supply device is turned OFF.
That is, the field decay surge is generated when the switch with respect to the inductive load in an electrically conducting state is turned OFF, whereby the frequency of occurrence becomes higher than other surge voltages such as surge voltage resulting from ESD (Electrostatic discharge) and surge voltage flowing from other power supply systems, which adds to the problem of heat generation. In consequence, the efficiency may lower due to heat loss of the power supplied from the power supply device and failure of the power supply device may arise.